Conventional printers, for example commercial computer printers for use in the office or home for printing out images in general including photographs, drawings, text, web pages and the like, and industrial printers for printing e.g. billboards and display posters use a process of digital halftoning to convert electronic image data into print instructions sent to a printhead in order to print a plurality of small dots onto a printing medium e.g. paper, to create an image on the printing medium. Typically for text or line drawings, halftoning is not needed, because lines are printed solid. However, halftoning is used for printing images having varying shades of color or gray, such as photographs.
Halftoning is the transformation of a grayscale or color image to a pattern of small spots with a limited number of colors (e.g. just black dots on white background, or spots of the primary colors cyan, magenta and yellow), in order to make it printable. Printing is in its bare essence a binary process for each point on the paper: put ink (or tone) on a paper or leave the paper uncovered (e.g. white). This would suggest that only binary images could be printed. However, halftoning makes it possible to reproduce so-called continuous-tone images, which are images with different shades of gray or color. The halftone process creates patterns of dots on a background. When viewed from a sufficient distance, the human viewer will be unable to see the dots themselves, because they are too small. Instead, the human viewer will have the illusion of a gray or continuous-tone color. In gray-scale halftoning, only black dots are created, whereas in color halftoning dots of the three primary colors of subtractive color mixing, cyan, magenta, and yellow (=color separation) are created and printed on top of one another.
U.S. Pat. No. 6,505,905 describes a halftoning method in which continuous-tone values representing dots of an image are split into two or more pass values, e.g. in a relationship 60% to 40%. The pass values are then subjected to an error diffusion procedure which results in two or more independent sub-images induced by the pass values. In a first pass, e.g. from left to right, the result of the error diffusion of the 60%-value is printed, and in the second pass, the result of the error diffusion of the 40%-value is printed.
U.S. Pat. No. 6,193,347 refers to a hybrid multi-drop/multi-pass printing system, where the printing activity between the different passes is highly correlated and dependent.